The present invention relates to leak detection systems and particularly to methods and apparatus for calibrating and validating such systems.
Leak detecting instruments are used in a variety of industries to determine whether products are properly manufactured and assembled. Leak detecting instruments are used to test products for the presence of leaks which may degrade the performance of the product during the product's useful life. Not all leaks are fatal to the performance of a product and a maximum acceptable leak is often established for a product. Consequently, leak detecting instruments must be able to distinguish between leaks which fall above or below the maximum acceptable leak.
Because leak detecting instruments not only detect leaks, but measure the amount of leakage as well, leak detecting instruments are calibrated to accurately measure the amount of leak into or from a part being tested. Leak detecting instruments are therefore calibrated to a leak standard. The leak standard establishes a known flow rate or pressure change which is used to calibrate the sensing apparatus of the leak detection equipment measuring leakage, thereby ensuring that the leak detection equipment accurately measures the amount of leak present in the parts being tested.
The basis of known dry-air leak detection instruments is either a pressure-difference sensor or a mass-flow sensor. Examples of such systems are shown U.S. Pat. No. 5,546,789 and U.S. Pat. No. 6,279,383 which are assigned to the assignee of the present invention. For quality assurance purposes, the correct performance of the instrument (not simply of the pressure or flow transducer) is validated periodically against some external standard. The most common method of accomplishing this is to connect a “calibrated leak”, i.e., a small constriction or orifice passing a fixed leak flow at the required test pressure, in parallel across a known-good test part. Leakage through the test part, if any, is identified as “zero” leak and is off set during the leak detector calibration process. A leak detection instrument which shows a leak rate corresponding to the calibrated leak is then considered validated. Another method, applicable to pressure-difference systems, is to arrange for a small piston to be moved in a sealed cylinder through a known stroke volume as shown in U.S. Pat. No. 4,811,252. An improved method is to use a calibrated precision flow sensor in series with an adjustable needle-valve in place of the constriction, the valve being adjusted by hand until the desired leak flow appears as shown in U.S. Pat. No. 5,363,689.
Although each of the above systems has proven to be valuable they exhibit drawbacks in production use. For example, the pneumatic resistance of a small constriction or orifice varies with air temperature and density, and is liable to change over time as it becomes partially blocked by contamination or moisture. Also, the piston of a pressure type detector moving through a stroke volume may suffer from sticking problems as well as wear due to motion of the seals.
Further, a production supervisor may wish to set-up automatic validation of the leak detection instrument, perhaps once per part tested for safety-critical units, or at lesser intervals of once per shift or once per week for less demanding applications. Up to now, this required a solenoid valve to be opened by the automatic control at the correct time, to connect a standard leak into the leak-test system. Actuating the valve inevitably involves a change in the volume of the system, which in itself can disturb the leak measurement and introduce error in the validation, particularly in the testing of smaller parts.
A need exists for an improved reference source cell leak flow calibration source as well as an improved arrangement for fitting such a source into production testing equipment.